The present invention relates to a dielectric material for high frequency, and a dielectric resonator, a dielectric filter and a dielectric duplexer obtained with use of it.
Propagation of communications equipments utilizing a high frequency region such as microwave has been very remarkable in recent years. Dielectric ceramics came to be widely used as a materials for dielectric filters and circuit boards of the equipments used in this region.
The dielectric ceramics used here is required to have a high specific dielectric constant ∈r, a higher-unloaded quality coefficient Q and a lower temperature coefficient of resonance frequency.
Conventionally, many compositions has been known as such dielectric ceramic materials for high frequency. For example, a composition consisting of Ln2O3xe2x80x94Al2O3xe2x80x94CaOxe2x80x94TiO2 (Ln is a rare earth element) is disclosed as a material which has a specific dielectric constant of not lower than 30, an unloaded quality coefficient Q of not lower than 20000 at 1 GHz and can control the temperature coefficient of resonance frequency xcfx84f within xc2x130 ppm (Japanese Patent Publication No. 2625074 and Japanese Laid-Open Patent Publication No. 57537 of 1995).
Also, U.S.S.R Patent No. 590299 discloses that a ceramic composition based on LaAlO3xe2x80x94CaTiO3 comprising La as a rare earth element is useful in thin membrane condensers.
A composition consisting of LaAlO3 and CaTiO3 or a composition consisting of NdAlO3 and CaTiO3 is a material which has a specific dielectric constant xcfx84r of not lower than 30 and an unloaded quality coefficient Q of not lower than 20000 at 1 GHz and can control the temperature coefficient xcfx84f of resonance frequency within xc2x130 ppm. However, it has been difficult to get a material which can produce stably in a large scale a dielectric ceramic element superior in total balance and with a higher characteristic.
An object of the present invention is to provide a dielectric composition for high frequency, which has a specific dielectric constant xcfx84r of not lower than 40, an unloaded quality coefficient Q of not lower than 40000 at 1 GHz and can control the temperature coefficient xcfx84f of resonance frequency within xc2x120 ppm.
According to the present invention, the above object has been attained by using La and/or Nd as the rare earth element and iron as the additive.
Thus, the dielectric ceramic composition for high frequency according to the present invention comprises a main composition expressed by the general formula x(CayTiO2+y)xc2x7(1xe2x88x92x)(LnzAl2xe2x88x92zO3) where Ln is at least one rare earth element containing La and/or Nd, and 0.600xe2x89xa6xxe2x89xa60.730, 0.950xe2x89xa6yxe2x89xa61.050, 0.980xe2x89xa6zxe2x89xa61.030, and iron, which is added as Fe2O3 in an amount of 0.01 to 0.5 parts by weight to 100 parts by weight of the main composition.
Ln may consist of La or Nd each alone or may contain them together, or may contain at least one of them and another rare earth element. However, when the value of said x exceeds 0.730, the temperature coefficient xcfx84f of resonance frequency comes to be higher than 20 ppm and the unloaded quality coefficient Q at 1 GHz comes to be decreased to 30000 level or 20000 level and no desired effect can be obtained.
Reversely, when the value of x comes to be lower than 0.600, the temperature coefficient xcfx84f of resonance frequency comes to be negatively high and a specific dielectric constant ∈r of not lower than 40 cannot be obtained.
When the value of y is lower than 0.950 or exceeds 1.050, the specific dielectric constant ∈r comes to be low and a specific dielectric constant ∈r of not lower than 40 cannot be obtained and at the same time the fQ value is also lowered and a fQ value of not lower than 40000 cannot be obtained. When the z value is lower than 0.980 or exceeds 1.030, similarly the specific dielectric constant ∈r and the fQ value are lowered and no desired effect can be attained.
Furthermore, the present invention is characterized in that iron is added. The added amount may be in a ratio of 0.01 to 0.5 part by weight as Fe2O3 to 100 parts by weight of the main composition. When the added amount exceeds 0.5 part by weight, the fQ value cannot be increased. When it is lower than 0.01 part by weight, a fQ value not lower than 40000 cannot be obtained.
In the present invention, as far as the object is not impaired, additives such as SiO2, MnO2, NiO, CuO, B2O3 and Li2CO3 may be added.
A dielectric resonator of high performance can be prepared by using such a composition according to the present invention. Said resonator is prepared by firing the ceramic composition as mentioned above.
The content of impurities at the stage before calcination is important. Particularly, it is highly specific that SO4 anion largely affects the deterioration of the dielectric material characteristics and the inhibition of sintering.
Accordingly, it is required to use raw materials of high purity. In the present invention, lanthanum oxide and/or neodymium oxide and calcium carbonate used as the raw materials are dissolved in nitric acid and the nitric acid-insoluble materials contained in the raw materials is filtered off to remove most of SO4 anion so that a product of high performance can be obtained.
Furthermore, uniform mixing of raw materials is desired at the stage before calcination. In the present invention, lanthanum oxide and/or neodymium oxide and calcium carbonate are dissolved in nitric acid, other raw materials (titanium oxide and aluminium oxide) are mixed together and then the mixture is pulverized until an average particle size is not higher than 0.5 xcexcm, preferably not higher than 0.4 xcexcm, to enable stable uniform mixing.
Though ferric sulfate (and nickel oxide) may be added from the beginning, it is preferred that it is added after calcinating the main composition, the mixture is pulverized, and the whole is mixed uniformly and then fired. A binder such as polyvinyl alcohol may be used for molding before firing.
The following examples serve to illustrate the invention in more detail although the invention is not limited to the examples.